It is known that the shape memory phenomenon consists in the fact that a mechanical piece made of an alloy that exhibits said phenomenon is capable of transitioning, upon a temperature change, between two shapes that are preset at the time of manufacturing, in a very short time and without intermediate equilibrium positions. A first mode in which the phenomenon may occur is called “one-way” in that the mechanical piece can change shape in a single direction upon the temperature change, e.g. passing from shape A to shape B, whereas the reverse transition from shape B to shape A requires the application of a mechanical force.
On the contrary, in the so-called “two-way” mode both transitions can be caused by temperature changes, this being the case of the application of the present invention. This occurs thanks to the transformation of the micro-crystalline structure of the piece that passes from a type called martensitic, stable at lower temperatures, to a type called austenitic, stable at higher temperatures, and vice versa (M/A and A/M transition).
A SMA wire has to be trained so that it can exhibit its features of shape memory element, and the training process of a SMA wire usually allows to induce in a highly repeatable manner a martensite/austenite (M/A) phase transition when the wire is heated and to induce an austenite/martensite (A/M) phase transition when the wire is cooled. In the M/A transition the wire undergoes a shortening by 3-5% which is recovered when the wire cools down and through the A/M transition returns to its original length.
This characteristic of SMA wires to contract upon heating and then to re-extend upon cooling has been exploited since a long time to obtain actuators that are very simple, compact, reliable and inexpensive. In particular, this type of actuator is used in some bistable electric switches to perform the movement of a drive element from a first stable position to a second stable position and vice versa. It should be noted that the term “drive element” is intended here to have a very generic meaning since it can take countless shapes according to specific manufacturing needs, as long as it is the element whose movement determines the commutation of the switch between two operating positions, i.e. the opening and closing of an electric circuit.
Some examples of this specific application of SMA wires are described in U.S. Pat. Nos. 4,544,988, 5,977,858 and 6,943,653. The several different embodiments illustrated in these patents share the use of a pair of opposing SMA wires to push a drive element between two stable positions. It should be noted that since the small run that can be obtained from the shortening of a SMA wire would be insufficient to cover the entire run between the two stable positions, said SMA wire is used only to move the drive element through a distance sufficient to arrive beyond the dead center of a snap-action spring connected to said drive element and suitable to take it up to the end of the run.
A typical example of a snap-action spring is a leaf spring secured at its ends such that it remains compressed and toggles between two stable symmetrical positions, as illustrated in the above-mentioned patent U.S. Pat. No. 5,977,858. In the present description, reference will be made to a similar arrangement while it is clear that other types of snap-action springs can be used, such as those disclosed in the other patents U.S. Pat. No. 4,544,988 and U.S. Pat. No. 6,943,653.
The above-mentioned known embodiments share the feature of having two SMA wires permanently connected to or in contact with the drive element on which they act, and this implies a double drawback.
In the first place, the SMA wire that is activated (i.e. that is heated to contract) must exert on the drive element a force not only sufficient to overcome the resistance of the spring to make it snap to the other stable position but also sufficient to tension the other SMA wire that is not activated yet is in contact with the drive element. In other words, the force exerted by the activated SMA wire is partially used to tension the other SMA wire that is moved together with the drive element.
Secondarily, the SMA wire that is not activated undergoes however a mechanical stress that over time may cause fatigue problems in the material. As a consequence, at each operating cycle of the switch both SMA wires are stressed: the activated wire for its normal shortening and re-extending cycle and the wire that is not activated for the mechanical stress received through the drive element.